The present invention relates to optical windows, and especially to windows for visible and near infrared transmission intended for use in airborne reconnaissance and surveillance systems.
Airborne systems of this type, which may use infrared sensors, TV imagers, and laser designators, for example, require windows with good optical transparency over the range of wavelengths utilized (approximately 0.5 to 12 .mu.m). Broadband window materials now available for this purpose, however, are sensitive to moisture and are especially subject to erosion and other damage by high velocity raindrops striking the surface. This, of course, is a frequent environmental condition in airborne service, and many materials having excellent transparency at the desired wavelengths, and other desirable properties, are not acceptable for use in airborne optical windows because of the rapid erosion by rain or moisture which causes progressive loss in optical transmission, as well as other mechanical damage, or even catastrophic failure. Other materials are available which have good resistance to rain erosion and other damage under these severe conditions of airborne use but which have poor optical transmission in the infrared spectral range, especially in the relatively thick sections required for adequate mechanical strength, so that these materials are not suitable for use as airborne windows.
It has been proposed to provide protective surface coatings on optical windows to protect them from erosion by raindrops so as to permit the use of materials of good optical properties. Thus, it has been suggested that materials such as barium fluoride and sodium chloride which have good optical transparency in the infrared but are very moisture sensitive may be protected by coatings of such materials as calcium fluoride, lanthanum trifluoride and polytetrafluoroethylene which have good moisture resistance. Such coatings are applied by vacuum deposition, that is, by vaporization or sputtering of the coating material in a vacuum to deposit a film or coating of the desired thickness on the substrate. Such vacuum deposited coatings by themselves, however, are not satisfactory because of their susceptibility to cracking and propagation of scratches, as well as having a tendency to separate from the substrate.